The current semiconductor device usually has a very high integrity. Consequently, in order to prevent the entire semiconductor device from becoming a defective product because of a partial defective circuit, a redundant circuit is formed in advance so that the defective circuit detected by examination can be switched to the redundant circuit.
In general, fuses are prearranged in the semiconductor device. When it is necessary to switch a defective circuit to the redundant circuit, the fuse corresponding to the defective circuit is cut off by a laser beam, followed by programming according to the state of the fuses. In this way, the defective circuit can, in practice, be switched to the redundant circuit.
However, the wiring width becomes narrower to accompany the improvement in the integrity of the semiconductor device. On the other hand, the spot size of the laser beam cannot be reduced. As a result, the fuses cannot be arranged close to each other.
Consequently, the technology using antifuses in a semiconductor device has attracted much attention in recent years. According to this technology, programming is performed after the antifuse corresponding to a defective circuit is shorted by applying a prescribed voltage.
FIG. 18 shows an example of the conventional antifuse. As shown in the figure, lower wiring 211 and upper wiring 212 on an insulating film formed on the surface of the lower wiring are connected to circuit modules 221 and 222, respectively. Each crossing part of lower wiring 211 and upper wiring 212 is used as an antifuse 205. When a prescribed breakdown voltage (5-20 V) is applied to an antifuse 205 corresponding to a defective circuit, the insulating film sandwiched between lower wiring 211 and upper wiring 212 in the portion of said antifuse 205 is punctured. As a result, a short circuit is formed between lower wiring 211 and upper wiring 212, and the desired circuit blocks in circuit modules 221 and 222 are connected to lower wiring 211 and upper wiring 212, respectively. In this way, the problem caused by the defective circuit can be solved.
Symbol 251 in FIG. 19 represents a diffused layer formed in silicon substrate 250. The two ends of the diffused layer are connected to film wiring 241. An ONO film (oxide/nitride/oxide film) 253 is formed as an insulating film on diffused layer 251, and a polysilicon film 252 with its two ends connected to film wiring 242 is formed on said ONO film 253. An antifuse 245 is formed at the intersection of diffused layer 251 and polysilicon film 252.
FIG. 20 shows a cross-sectional view along line A--A of said antifuse 245. When antifuse 245 is shorted, a prescribed voltage is applied between two wirings 251 and 252 to puncture ONO film 253 between diffused layer 251 and polysilicon film 252. As a result, a short circuit is formed between wirings 251 and 252.
There is no need to use a laser beam if electrically shorted antifuses are used as described above. Consequently, the antifuses can be arranged close to each other so that the area occupied by the chip can be reduced.
Unlike the fuse cut off by the laser beam, the punctured surface of the antifuse is not exposed to the surface of the semiconductor device. Therefore, a highly-reliable semiconductor device can be obtained because moisture and impurities cannot enter the punctured surface.
However, when said antifuses 205 and 245 are used in the semiconductor memory device, it is necessary to use a special-purpose film for forming antifuses 205 and 245 in a process separate from the process for forming the memory cell and the peripheral circuit. Consequently, the manufacturing cost is increased, while the yield drops as a result of using more films.
An object of this invention is to solve the aforementioned problems of the conventional technology by providing a semiconductor memory device having antifuses without adding a film manufacturing process.